1. Field of the invention
The present invention relates to a transistor protection device, more particularly to a transistor protection device for protecting an air-flow controlling transistor for an automobile air conditioner by radiating the heat accumulated in the transistor.
2. Description of the Prior Art
In general, there are various kinds of transistors incorporated in an electrical circuit of an air conditioning device of automobile. Among them, there is in particular a power transistor with relatively large capacity, which is used for adjusting the air-flow amount of a fan through control of the rate of rotation of a fan motor. Namely, the power transistor is adopted for controlling the speed of the fan motor to adjust the flow amount of the air.
The power transistor is installed in the control circuit, and when the fan switch is set at the auto position, an automatic amplifier detects the voltage at point A which is generated in response to the difference between the resistance of PTC (patentio-thermo control) corresponding to a set temperature and the resultant resistance detected by the interior and exterior temperature sensors. The automatic amplifier further detects the voltage at point C which is generated in response to the difference between the two fixed resistances to very the voltage at the base of the power transistor stepwise or continuously in order to change the rotation speed of the fan motor, resulting in controlling the air-flow amount automatically.
However, the power transistor is made of a semiconductor so that it produces heat when an electric current flows through it. Therefore, in the prior art devices, the power transistor is attached to the bottom wall of a radiator which is called a heat sink which is placed in an air duct of an automobile air conditioning device to be cooled forcibly by the air that flows in the duct.
However, in a transistor protection device described above, the radiation efficiency is not sufficiently high, resulting in a decrease in the function, and a shortening of the life, of the transistor.
In addition, since the portion of highest heat production in the power transistor is the bottom portion, it tends to cause a local deterioration of the transistor.